1. Field of the Invention
The present invention relates to power wrenches and, more specifically, to power tongs for making up and breaking out threaded connections between adjoining tubular members. In particular, the invention relates to a novel arrangement used to join the tong cage plates and the rotary gear for joint rotation to facilitate proper alignment of the tong parts for placing the tong on the pipe or removing the tong from the pipe. The novel arrangement also serves the novel function of preventing so-called "overcamming" situations wherein the pipe gripping mechanism of the tong does not grip the pipe due to various factors, such as the jaws undersized or the pipe being undersized.
2. Description of the Prior Art
Oil field tubular members, e.g., drill pipe and casing, are employed in sections which are jointed together at their ends by threaded connections. Power tongs of the type herein described are utilized to make up and break out these threaded connections by securely gripping one tubular member and rotatably driving that member relative to the adjoining member. Tongs representative of present practice in the industry are described in U.S. Pat. Nos. 2,879,680; 3,180,186; 3,261,241 and 4,084,453.
When a power tong is used to grip and rotate a pipe section, a pipe-gripping mechanism as described, for example, in U.S. Pat. No. 2,879,680, is utilized to bring a pair of jaws into contact with the pipe. The jaws are pivotally mounted by bolts which pass through the jaws. The bolts are secured at their ends to the so-called "upper and lower cage plates". The cage plates are generally arcuate plates that are adapted to rotate with the jaws after the pipe has been securely gripped. The jaws are movable into and out of engagement with the pipe section by means of a camming action caused by jaw rollers (cam followers) which travel on machined cam surfaces of a rotatably driven rotary gear.
In pipe gripping operations, the jaws (or dies carried thereon) may fail to securely grip the pipe due to various factors. In such cases it is desirable to refrain further relative movement between the rotary gear and cage plate to prevent dangerous overcamming situations where the rotary gear continues to rotate after the camming action fails. No known tong structure adequately solves this problem.
A second problem in tong operation addressed by the instant invention is that of alignment of the tong parts prior to placing the tong on the pipe or removing the tong from the pipe. In an open mouth tong of the type described herein, three openings must be aligned before the tong may be placed on or removed from a pipe-- the tong throat, the rotary gear mouth and the cage plate mouth. According to prior art practice as illustrated in FIGS. 7 and 8, this alignment function has been accomplished by the use of a backing lug/backing pin arrangement. According to this prior art technique, a "backing lug" 200 is secured to the inside surface of the rotary gear 230 by means of a fasterner 202 and protrudes toward the center of the tong. The backing lug 200 cooperates with a backing pin 210 which may be placed into either of two openings in the upper cage plate 225 (opening 205 shown in FIG. 7). In the operation of this well-known prior art assembly in a make up operation, after the casing has been made up to the desired torque the rotary gear is driven in the reverse direction to disengage the jaws and rotates in that direction until the backing lug 200 engages the backing pin 210. At the point of this engagement the mouths of the rotary gear and cage plates are aligned and with continued rotation of the rotary gear and cage plates may be rotated to the point where the aligned mouths align with the throat of the tong. At this time all three openings are aligned and the tong may be removed from the pipe. A reverse operation is utilized for removing the tong after a break out operation.
A problem associated with the described prior art technique is that in situations where the rotary gear is rotating at a relatively fast rate of speed (for example, when the rong is in high gear and the throttle valve is completely open) the force exerted at the instant the backing lug contacts the backing pin may result in a sheared backing pin or a sheared backing lug, or both. Such situations are costly in terms of time of replacement and repair since the tong must be partially disassembled to replace a backing lug and due to the further problem that a broken pin may fall downhole and cause problems for rig operation personnel.
The described alignment technique has proved moderately successful in practice, but has caused the noted problems with reliability and tong downtime. It is an object of the present invention to overcome the disadvantages associated with this and other prior art techniques and to provide means for preventing costly and dangerous overcamming situations.